The present invention relates to the preparation of thin films by magnetron sputter deposition in which the angular distribution of sputtered particles emitted from the target and arriving (depositing) at the substrate is directional. Directional emission and arrival mean that the angular distribution of flux intensity of sputtered particles emitted from the target, and incident at the substrate are each characterized by a narrow peak or peaks on a low level background angular distribution. In other words, the majority of particles emit and arrive at about the same one or few narrow ranges of angles. The directional emission is preserved by ballistic transport to result in the directional arrival.
One of the most important commercial processes for depositing thin films of a desired material onto a substrate is sputter deposition, also known as sputter coating or sputtering. Sputter deposition is used extensively in many industries including the microelectronics, data storage and display industries to name but a few. Generally, the term sputtering refers to an xe2x80x9catomisticxe2x80x9d process in which neutral, or charged, particles (atoms or molecules) are ejected from the surface of a material through bombardment with energetic particles. A portion of the sputtered particles condenses onto a substrate to form a thin film. The science and technology of sputtering is well known and described for example in Vossen, J. L., Kern, W., Thin Film Processes, Academic Press (1978). Sputtering can be achieved through several techniques. Generally, in xe2x80x9ccathodicxe2x80x9d (xe2x80x9cdiodexe2x80x9d) sputtering the target is at a high negative potential relative to other components, usually through application of a negative bias from a power supply, in a vacuum chamber system, typically containing an inert gas or mixture of gases at low pressure. A plasma containing ionized gas particles is established close to the target surface and ionized gas particles are accelerated by the action of the electric field towards the target surface. The bombarding particles lose kinetic energy through momentum exchange processes with the target atoms, some of the latter particles gain sufficient xe2x80x9creversexe2x80x9d momentum to escape the body of the target, to become sputtered target particles. Note a sputtered particle may be an atom, atom cluster or molecule either in an electrically neutral or charged state. A flux of sputtered particles may contain any one or any mixture of such entities.
One category of sputtering processes is known as magnetron sputtering. Magnetron sputtering is the most widely used form of sputtering and is the mainstay of commercial sputter deposition processes. In magnetron sputtering, crossed electric and magnetic fields generated by a magnetron assist in the sputtering by concentrating sputtering action.
According to the known art, the sputtered particles from a typical magnetron sputter cathode source emit with a cosine distribution or some variant based upon it. See Wasa, K., Hayakawa, S., Handbook of Sputter Deposition Technology, Noyes Publications, 1992. Film thickness distributions generally reflect that of calculations based upon a cosine emission model. See Vossen, J. L., Kern, W., Thin Film Processes, Academic Press (1978); see also U.S. Pat. No. 5,417,833 to Harra et al. Typically in many commercial sputter processes, sputtered particles are incident at the substrate at angles far from normal incidence even under ballistic transport conditions. See Rossnagel, S., J. Vac. Sci. Tech. B., Vol.16, No.5, p. 2585 (1998). This effect is desirable if the substrate features to be coated have low aspect ratios. However, many leading edge technological applications involve, for example, filling deep, sub-micron high aspect ratio trench or via structures or coating high aspect ratio features with a high degree of conformality. However, there are limits on the smallness of the critical dimension of such features that can be conformally covered or filled by PVD.
A directional sputter deposition technology based upon magnetron sputtering (further described below), in which the angular distribution of sputtered particles incident at a substrate/thin film growth surface could be xe2x80x9ctailoredxe2x80x9d to the particular requirements of the thin film application, would be of significant technical and commercial value with a wide scope of technological application. Such a technique may allow the following to be improved: film coverage, engineering and control of thin film microstructure and therefore related functional characteristics of the film. For example, directional deposition would greatly ease step coverage of high aspect ratio features, e.g., channels on patterned surfaces. However, methods proposed to improve directionality, while providing some benefits, need further improvement.
Coating high aspect ratio structures is of critical importance, e.g., in emerging submicron semiconductor interconnect metalization and high density data storage media applications. In such cases the bounds of application of magnetron sputter deposition is approaching its limit. For example, in coating via type structures in microelectronics interconnect applications, it is well known that sputter deposition suffers from film buildout at the upper edges of the via resulting in a trapped void, xe2x80x9ckeyholexe2x80x9d type film defect as well as other film defects. See, for example, Rossnagel, S., J. Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998). This effect is exasperated with reducing dimensionality and increased aspect ratio. Proponents of current commercial PVD processes assert they can conformally cover relatively high aspect ratio features, or fill relatively high aspect ratio channels or vias, having a critical dimension of at least 0.18 micron, or perhaps greater than 0.13 micron.
Several sputter PVD techniques, many of them developed commercially relatively recently, attempt to control the directionality of the incident sputtered particle flux at a substrate e.g., physical collimation techniques, hollow cathode sputtering, arc sputtering, self ionized sputtering, ionized physical vapor deposition (IPVD) and long throw methods. The latter two techniques probably represent state of the art commercial technologies. The scope, scalability, efficiency and cost considerations of directional sputter technologies have been reviewed by Rossnagel, S., J. Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998). The best techniques utilize tooling and/or process attributes to achieve a degree of control over the angular distribution of incident sputtered particles. These methods are in fact expressly designed to overcome what are believed to be inherent deficiencies in basic magnetron cathode sputter deposition characteristics and target materials design which limit control of the substrate incident angular sputtered flux distribution.
For example, in IPVD techniques a coil is located in the vacuum chamber between the sputtering cathode and substrate on which the film is to be deposited. The coil is configured to form a secondary plasma in the region above the substrate. The magnetron sputtered particles pass through a relatively high pressure ambient for creating the desired secondary plasma to undergo significant gas phase scattering, ionization (partial) in the secondary plasma followed by electrostatic deflection towards the substrate surface, generally provided by electrically biasing the substrate. At the substrate, partial resputtering of the growing film by the electrostatically accelerated particles is used to control film characteristics. For example bottom and sidewall coverage in semiconductor interconnect applications. Clearly complex post sputter emission processes are central to the directionality and the degree of conformal coverage achieved by the IPVD technique.
U.S. Pat. Nos. 5,948,215; 5,178,739; and Patent Cooperation Treaty published application No. WO 98/48444 disclose ionized plasma vapor deposition processes, and are incorporated herein by reference.
Long throw methods utilize ballistic (i.e., collisionless) transport and a long throw path to the substrate to xe2x80x9copticallyxe2x80x9d filter the magnetron cathode emitted flux such that only relatively low angle components of the emitted flux (i.e., those close to the target normal) are incident at the substrate. The long throw process is clearly inefficient through flux dilution and suffers from inherent asymmetries in the incident flux. See Rossnagel, S., J. Vac. Sci. Tech. B., Vol.16, No.5, p. 2585 (1998).
Directional sputter emission from single crystals, see Werner, G. K. Phys. Rev. J. Appl. Phys., Vol. 26, p. 1056 (1955) and ibid., Vol. 102, p. 690 (1956), and polycrystals, see Werner, G. K., Rosenberg, D., Phys. Rev. J. Appl. Phys., Vol. 31, No. 1, p. 177 (1960), has been known for many years from work in diode sputter systems (not magnetron systems). However, referred emission has remained a laboratory result and has not been put into substantive practical application. This is probably due to several factors e.g., the cost and practical difficulty of producing large single crystal targets, together with problems due to reproducibility of the effect and the dominance of the magnetron sputter cathode in commercial applications of sputtering (The rate of deposition in diode sputtering is generally non-viable for many commercial applications). Initial experiments studying sputter emission from polycrystalline targets using diode sputtering techniques, see Werner, G. K., Rosenberg, D., Phys. Rev. J. Appl. Phys., Vol. 31, No. 1, p. 177 (1960), did not show directional effects in several elemental systems. Directionality has been observed for sputtering of crystallographically textured polycrystalline targets using angularly collimated, monoenergetic ion beams. See Tsuge, H., Esho, S., J.Appl. Phys., Vol. 52, No.7 (1981), Smith. P. C. et al, J. Vac. Sci. Technol. a 17(6), p. 3443, November/December 1999. Factors inherent in the design and operation of magnetron sputter cathodes differentiate them from diode sputtering and sputtering through angularly collimated, monoenergetic ion or particle beams which affect the sputtered particle emission profile. For example, in contrast to magnetron sputtering there is no racetrack in diode sputtering, neither is the sputtering flux monoenergetic as in ion beam studies, see for example Vossen, J. L., Kern, W., Thin Film Processes, Academic Press (1978). Magnetron sputtering of crystallographically textured targets has been used as a means xe2x80x9cto point the emission distribution in a more normal distributionxe2x80x9d. See Rossnagel, S., J. Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998). It has not however been sufficiently developed for technological application as a directional technique as demonstrated by the pre-eminence of IPVD and Long Throw techniques. See Rossnagel, S., J. Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998).
In general, magnetron sputtering uses crossed electric and magnetic field configurations to concentrate the sputtering action. Generally a negative bias is applied to the target, hence the magnetron and target assembly, which form the basic elements of the sputter source, is referred to as the cathode or magnetron sputter cathode. Typically, but not exclusively, magnets are positioned behind the sputter target. Magnetic field lines penetrate the target, threading through the low-pressure gas environment above the target before re-entering the target body. The configuration of crossed electric and magnetic fields are designed to confine electrons emitted through the bombardment of the target by energetic gas phase ions (and/or atoms) and increase the effective path length of ionizing electrons. A drift velocity is imparted on the electron motion. Their net motion describing a closed loop or so called xe2x80x9cracetrackxe2x80x9d. The overall effect is to increase the efficiency of ionizing the process gas and therefore the density of ions in the plasma. The consequent increased target bombardment enhances the efficiency of the sputtering process.
Both fixed and movable magnet structures have been utilized in magnetron sputtering. In one prior art sputtering system utilizing a moving magnet, the target is circular and the magnet structure rotates with respect to the center of the target. In a second prior art sputtering system utilizing a moving magnet, the target is rectangular or square and the magnet structure is scanned along a linear path with respect to the target. In a third prior art sputtering system, the target is rectangular and the substrate is moved in a plane parallel to the surface of the target during sputtering. The second type of sputtering system is known as a linear scan sputtering system and disclosed, for example, in U.S. Pat. No. 5,382,344 to Hosokawa et al and U.S. Pat. No. 5,565,071 to Demaray et al, both of which are incorporated herein by reference in their entirety.
Linear scan sputtering systems typically utilize an elongated magnetron assembly that produces a correspondingly elongated closed-loop racetrack plasma profile. The magnetron (magnet) assembly is reciprocally scanned parallel to the target surface and perpendicular to the long dimension of the magnetron assembly. As the magnetron assembly is scanned with respect to the target, the plasma follows the instantaneous position of the magnetron assembly, as a scanning racetrack, and sputters areas of the target. The magnetron is typically behind the target assembly, but other configurations are possible. The target assembly may include a sputter target and backing plate or be of monolithic construction, that is, the sputter target and backing plate are formed from a single piece of material. This assembly may include several possible elements in addition to the sputter target and backing-plate, for example, possibly a heat exchanger assembly. Considerations of the total thickness and magnetic characteristics of these components and the spacing between the magnetron and the target assembly will have a significant impact on the intensity of sputtering produced and therefore deposition rate. Erosion anomalies are typically observed in linear scanning systems at the scan amplitude extremities. Importantly, in conventional linear scanning systems such erosion anomalies are generally undesirably reflected to some extent in the film thickness distribution and also lead to inefficient utilization of the sputter target, reducing its service life through localized enhanced wear. U.S. Pat. No. 5,855,744, incorporated herein by reference in its entirety, asserts a system to compensate for the effects of erosion anomalies.
An object of the present invention is to provide a sputtering process having directional sputter emission, ballistic transport, and directional arrival of sputtered particles (ions and/or neutrals) at the substrate/film growth interface.
Another preferred object of the present invention is to provide uniformly homogeneously coated substrates.
Another preferred object of the present invention is to provide devices for use in sputtering.
Another preferred object of the present invention is to sputter from a target such that the maximum angle of peak particle emission from the target is below a prespecified angle with respect to the target normal (on an unsputtered, planar target).
Another preferred object of the present invention is to enhance directional sputter emission by providing a method employing cathode operating parameters to reduce the intensity of the influence of high angle components, i.e., sputtered material emitted from the target at a high angle away from the global maximum of the sputtered material flux, relative to desired low angle components.
The present invention relates to a new sputter deposition technology that offers advanced control over the substrate incident angular sputtered flux distribution with process simplicity. This technology differs from other techniques in that controlled sputter emission from the magnetron sputter source, based upon improved materials and cathode design, is central to the technique. The cathode comprises the target and the magnet assembly of the magnetron. Specifically, the technology is a combination of (1) engineered materials design and preparation of the sputter target to promote directional sputter emission, (2) the nature, design and operation of the sputter cathode, and (3) the process, in which sputtered particle transport is essentially ballistic.
These elements in proper combination and use largely preserve beneficial attributes of the engineered directional emission characteristics of the sputter target materials through ballistic transport of the emitted particles and simple geometric considerations, which promote a high degree of directionality to the substrate incident sputtered particle flux.
Directional emission refers to an angular distribution of as-emitted sputtered particles whose flux intensity is characterized by a distribution of particles in which the majority of emitted particle flux is contained within a narrow peak, or peaks, superimposed upon a low level background angular distribution.
Thus, ideally, most of the flux arrives at the substrate at about the same one or few narrow ranges of angles most characteristic of emission from the target material. In particular, the particles are emitted from the target over a range of angles such that there is at least one local maximum of particles emitted at a respective angle in the range, and the total number of particles emitted within plus or minus 20xc2x0 of the respective angle of each local maximum constitutes a majority (preferably 80% or more preferably 90%) of the emitted particles. Preferably, the particles are emitted from the target over a range of angles such that there is a global maximum of particles emitted at a respective angle in the range, and the total number of particles emitted within plus or minus 20xc2x0 of the respective angle of the global maximum constitutes a majority (preferably 80% or more preferably 90%) of the emitted particles.
For example, the majority of sputtered material (preferably at least 80 or at least 90% of the sputtered material) may be emitted within a distribution of less than plus or minus 20xc2x0 of the first peak plus the sum of the material less than plus or minus 20xc2x0 of the second peak totals to be the majority of the sputtered particles.
Preferably, regardless of whether there are one or more peaks, the majority of the emitted particles are emitted within a total range of at most about 40xc2x0, more preferably at most about 30xc2x0.
For example, the process may be designed such that for a target material designed to have emission at a particular angle, most of the particles will emit at or about that angle. Thus, by properly positioning the substrate relative to the target, the majority of the emitted particles will impact the substrate at or about 90xc2x0. This makes it much easier to uniformly coat high aspect ratio features on the substrate.
The energetics of the majority of incident particles are generally insufficient to cause significant resputtering of the growing film.
The present invention relates to a directional thin film magnetron sputter deposition process for any directionally emitting target material or surface comprising, employing a scanning sputtering device to incrementally sputter material from the target material. The process desirably achieves uniform erosion of the sputter target over the target material life. Thus, an advantage of the process is that directional emission from the target, ballistic transport, and directional arrival at the substrate of sputtered material is effected and maintained.
To facilitate ballistic transport, the substrate is positioned at, or less than, the process free path of the majority of emitted particles from the emitting target surface.
In particular, the substrate is positioned, with respect to the sputter target, such that the majority (preferably at least about 80%, more preferably, at least about 90%) of emitted particles from the directionally emitting target surface experience ballistic transport across the space between the target and the substrate. This results in directional arrival of a majority of the emitted particles at the film growth surface of the substrate.
By applying at least a suitably sufficient power density during sputtering, directional emission can be obtained from a wider variety of target materials. In particular, raising power density can further reduce the intensity of the influence of high angle components relative to low angle components. The high angle components are defined as sputter target material emitted at a high angle, e.g., more than plus or minus 40xc2x0, away from the global maximum peak of flux of material emitted from the sputter target. Preferably, the low angle components are sputter target material emitted within plus or minus 20xc2x0 of the global maximum peak. When the global maximum peak is significantly away from being normal (90xc2x0) to the target surface, the target (or substrate) may be tilted to cause the emitted sputtered material to arrive at a desired angle. Unexpectedly, the high power removes unwanted potential high angle components.
Sputtering may also be through an ionized gas phase target species or so-called self sputtering in which (unlike IPVD with a secondary plasma) a self sustaining sputtering plasma is formed from the target species, requiring little or no plasma support gas once initiated.
The present technique may also be suitable for reactive sputtering. Reactive sputtering is generally described by Vossen, et al, Thin Film Processes, Academic Press (1978), incorporated herein by reference. Typically, in reactive sputtering a reactive gas or gases is added to an inert gas such that the plasma contains reactive species allowing the formation of compound thin films. Reactive gases can include for example oxygen, nitrogen, methane, hydrogen sulfide, hydrogen, carbon monoxide, etc. as is well known in the art.
In its device aspects, the present invention provides a device for magnetron sputtering. In particular, the device employs a magnetron housing and its sputtering process housing, the two housings being separated by a target, its backing and a cooling panel, wherein there is a low difference in pressure between the two housings.